Nitric oxide plays an important role in the regulation of biochemical pathways in living organisms. The inhalation of low levels (20 to 100 ppm) of nitric oxide has been shown to have a major therapeutic value in treatment of a diverse range of disorders ranging from reversible and irreversible pulmonary hypertension to treatment of neonates exhibiting hypoxemic respiratory failure and persistent pulmonary hypertension. Conventional medical uses of nitric oxide gas can involve dilution of a nitric oxide gas stream with gases immediately before administration of the nitric oxide gas to a mammal. Precise delivery of nitric oxide at therapeutic levels of 20 to 100 ppm and inhibition of reaction of nitric oxide with oxygen to form toxic impurities such as nitrogen dioxide gas is needed for effective inhalation therapy.